Generally, a pulse combustor includes a combustion chamber, an inlet for admitting combustion reactants (typically fuel and air) into the combustion chamber, and an outlet for expelling combustion products from the combustion chamber. Pulse combustors operate cyclically in that a charge of combustion reactants is admitted into the combustion chamber and ignited to form the combustion products, the initial ignition being assisted, preferably by a spark plug. The combustion products expand through the outlet thereby causing a partial vacuum in the combustion chamber which vacuum assists in drawing a fresh charge of combustion reactants into the combustion chamber for the next cycle. The fresh charge ignites upon mixing with the combustion products from the previous cycle, so that the operation is self-sustaining after the initial ignition.
Compared to conventional combustion systems, pulse combustors have the following attractive characteristics: two to three times higher heat transfer, an order of magnitude higher combustion intensity, one third lower emissions of oxides of nitrogen, 40% higher thermal efficiencies, and possibly self-aspiration. This combination of attributes results in favorable economic tradeoff with conventional combustors in many applications. Moreover, the enhanced heat and mass transfer associated with oscillating flow fields in pulse combustors may lead to significant improvements in industrial and chemical processes. Potential drawbacks of pulse combustors, however, are their inability to operate over a wide range of energy release rates (i.e., they have limited turn-down ratios), and their sensitivity to fuel properties, which may be highly variable geographically and temporally.
Although there are several different types of pulse combustors (i.e, the quarter-wave or Schmidt tube, the Rijke tube, the Helmholtz resonator, and the Reynst pulse pot), the underlying principle controlling their operation is the same, that is, the periodic addition of energy must be in phase with periodic pressure oscillations in order to sustain pulsations (Rayleigh's criterion). In other words, the energy release must be in phase with the resonant pressure wave. The phase relationship between the energy release and the resonant pressure wave is determined by four characteristic times of the pulse combustor system. The total delay time prior to energy release, and hence the phase relationship between the energy release rate and the resonant pressure wave, is a monotonically increasing function of three of the nearly independent characteristic times.
In spite of this apparent simplicity, the processes that occur in a pulse combustor are very complicated. They involve a three-dimensional, transient flow field that is highly turbulent and has variable physical properties. They further involve a resonant pressure field and a large transient energy release, the characteristic times of which may be on the same order of magnitude as the characteristic times for chemical reactions and fluid dynamic mixing. Moreover, all aspects of the combustion system are highly coupled.
The above-referenced characteristic times are the species mixing time, the fluid dynamic mixing time, the chemical kinetics ignition delay time, and the characteristic acoustic time; the first three of these characteristic times constitute the total ignition delay time.
There are many factors that, through their impact on these characteristic times, can affect the operational performance of a pulse combustor. For example, fuel properties, turn-down ratios, heat transfer, and equivalence ratios all have significant influences on the performance of a pulse combustor. However, of these times, the fluid dynamic mixing time scale appears to exhibit the strongest influence on the total ignition delay time, and thus performance of a pulse combustor. Because the various relevant time scales may all be comparable, and because the fluid dynamic mixing time is controllable, the fluid dynamic mixing time scale may be used to compensate for variations in other time scales as well as to achieve a desired operating condition.